Anyone who has swept an arm through the water has experienced the resistance to movement presented by a fluid. Though less dense than water, air is also a fluid and resists motion in exactly the same way. If you have ever indulged the unsafe temptation to extend an arm outside a fast-moving car you have experienced this force firsthand.

The study of objects moving through air is called aerodynamics. The resistance to the motion of an object through air or any other fluid is called drag. While aircraft designers have been studying drag for a century, automotive aerodynamics has only come into its own in recent decades. Discounting factors such as weight and horsepower, cars with superior aerodynamic characteristics accelerate more rapidly and handle better at high speed than their less aerodynamic competitors.

Faced with increasing fuel costs and environmental concerns, manufacturers of passenger cars have also become very conscious of fuel economy. Since over half the energy required to maintain a constant highway speed is expended to overcome air resistance, every car and truck is now designed with drag as a consideration.

There are many factors that determine drag, including the speed of the vehicle and the density of the air. But the factors under the control of the automotive designer are the size and shape of the vehicle.

The effect of shape on drag is measured by the drag coefficient. It is this quantity that engineers use to compare the aerodynamic efficiency of various body styles. The lower the drag coefficient, the more "slippery" the car, i.e., the less air resistance it encounters at high speed.

What makes some cars more slippery than others? Not surprisingly, rounded corners, inclined windshields, and sloping surfaces tend to produce less drag than "boxy" features. But the dynamics of air flow over a complex shape such as a car is very complicated, and it is not intuitively apparent which designs will outperform others.

To compare various options during the design process, engineers use software applications to model different shapes and calculate the drag coefficient of each. Modeling and simulation software has become so sophisticated that directly measuring drag coefficient in a wind tunnel is typically used only to validate the computational results prior to beginning production.

Most passenger cars exhibit a drag coefficient in the range 0.30 to 0.33. The unusual styling of the Toyota Prius has an impressive drag coefficient of only 0.26. By contrast, the drag coefficient of the blocky Hummer H2 is a whopping 0.57, among the highest of all production vehicles.

But recall that drag coefficient measures primarily the effect of shape on drag. The other design element is the size, or more specifically the frontal area, of the vehicle. This is the cross sectional area presented to the wind as the car moves forward. Multiplying the drag coefficient and frontal area represents the drag resulting from both shape and size. Comparing this "drag area," the supersized Hummer and other large SUVs remain the most aerodynamically inefficient vehicles. But the Prius takes a back seat to several subcompact cars that present even smaller frontal areas.

Reducing drag was obviously not an imperative for the Hummer. Surprisingly, it is not the primary consideration for race cars either. To enable the extreme acceleration and speeds needed by race cars, they are designed to be relatively lightweight. At very high speeds, so much air can flow under a race car that it may become airborne. This upward force is called lift. Generating a lot of lift is desirable for an airplane, but tragic for a car.

Thus, race cars and high performance sports cars are outfitted with air dams, spoilers, and/or wings. Air dams may block some air from flowing under the car to minimize lift. Wings and spoilers are shaped to use air flow to exert downward force, pushing the tires onto the pavement, counteracting lift, and improving high speed handling. These features collectively are not intended to promote aerodynamic efficiency and, as a result, race cars often exhibit higher drag coefficients than the car in your garage.

That car in the garage may have a spoiler on the trunk too. In some cars, a spoiler is used to smooth the air flow from the roof to the back of the vehicle, reducing air turbulence and drag at high speed. However, most spoilers on passenger cars are simply stylistic decorations, serving no functional purpose and often restricting visibility.

Large tractor-trailer trucks often employ a sloping spoiler or "dome" above the cab to smooth the transition to the trailer. This does not reduce the frontal area, but does create a far more aerodynamic shape. Reducing drag is especially important for commercial trucking, where fuel efficiency is key to economical operation.

Though we can reduce fuel costs by accelerating gradually, driving smoothly, and avoiding excessive speed, there is little we as drivers can do to impact the aerodynamics of our vehicles. One common misconception is that driving a pickup truck with the rear gate down will reduce drag. It will not, and may in fact even increase it slightly.

This is an example of automotive engineers designing with drag in mind. The closed gate helps create a cushion of relatively still air in the truck bed, over which air can flow more smoothly from the cab to the rear.

Drag is also dependent on a factor called skin friction. This is resistance caused by the surface texture of the car. As might be expected, a very smooth surface is slightly more slippery than a rough one. So taking some time to wax the car this weekend may yield some modest financial results as well as visual appeal.